Bug when using `.flatten()` method that has `Item = &'a T` when calling async function inside loop

I tried this code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f560d75b3eb32ab6d2e0e51590b0337f

I expected to see this happen: code compiles

Instead, this happened: error: implementation of std::marker::Send is not general enough

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rustc --version --verbose:

rustc 1.77.1 (7cf61ebde 2024-03-27)
binary: rustc
commit-hash: 7cf61ebde7b22796c69757901dd346d0fe70bd97
commit-date: 2024-03-27
host: x86_64-unknown-linux-gnu
release: 1.77.1
LLVM version: 17.0.6

Bug exists on beta and nightly versions also.

error: implementation of `std::marker::Send` is not general enough
  --> src/main.rs:63:5
   |
63 |     tokio::spawn(join_with_cancellation(f, cancellation.clone()));
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `std::marker::Send` is not general enough
   |
   = note: `std::marker::Send` would have to be implemented for the type `&CancellationToken`
   = note: ...but `std::marker::Send` is actually implemented for the type `&'0 CancellationToken`, for some specific lifetime `'0`

I have working example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=19183aa875e3dadf501f3b6bdfb61ce4.

It seems like the reason is combination of .flatten() method that has Item = &'a T when calling async function inside loop. If you try to remove .await from the line 33, it compiles. If you try to remove .flatten() as in second link - it compiles.

Also, the error seems kinda random. The pattern is as follows:

   = note: ...but `std::marker::Send` is actually implemented for the type `&'0 T`, for some specific lifetime `'0`

But every time I change the code, the compiler shows another T type in error hint.

Reproduce the failure from the Rust Playground links, starting at src/main.rs:63 and the async call near line 33. Compare the versions with and without .flatten() and .await, then trace the resulting Send lifetime diagnostic; done means the original example compiles correctly or the remaining behavior is isolated with a focused compiler regression test.